The present invention concerns a mass spectrometer.
In the following description, mass or m/z means a mass to charge ratio, and a mass range or a m/z range means a range for the mass to charge ratio.
In the linear ion trap mass spectrometer used for proteome analysis, etc., high sensitivity, high mass accuracy, MSn analysis, etc. are required. Mass spectrometry using the linear ion trap in the prior art is to be described.
In the prior art described, for instance, in U.S. Pat. No. 5,420,425 (Patent Document 1), after accumulation of ions introduced into an linear ion trap, ion selection or ion dissociation is conducted as required. Then, ions are ejected mass selectively from the linear ion trap in the radial direction by scanning a trapping RF voltage. It is described that the mass resolution is improved by superposing a supplemental AC voltage on quadrupole rods in this case. This enables mass analysis at high sensitivity.
In the prior art described in U.S. Pat. No. 6,177,668 (Patent Document 2), after accumulation of ions introduced into a linear ion trap, ion selection or ion dissociation is conducted as required. Then, ions are ejected mass selectively from the linear ion trap in the axial direction by applying a supplemental AC voltage on the quadrupole rods. Mass analysis at high sensitivity is possible by scanning the frequency of the supplemental AC voltage or the amplitude of the trapping RF voltage.
In the prior art described in U.S. Pat. No. 5,783,824 (Patent Document 3), after accumulation of ions introduced into a linear ion trap, ion selection or ion dissociation is conducted as required. Inserted lenses are interposed between quadrupole rods and a harmonization potential is formed on the linear ion trap axis by a DC bias between the inserted lenses and the quadrupole rod. Then, by applying a supplemental AC voltage between the inserted lenses, ions are ejected mass selectively from the linear trap in the axial direction. Mass analysis at high sensitivity is possible by scanning the DC bias or the frequency of the supplemental AC voltage.
Then, a method of measuring neutral loss scan or precursor ion scan in the prior art is to be described.
In a quadrupole time-of-flight mass spectrometer (QqTOF) or a triple quadrupole mass spectrometer (TripleQ), it has been proposed a method of conducting precursor ion scanning. For example, in the prior art described in ‘Organic Mass spectrometry, vol. 28, pp 1135 to 1143, 1993’ (Non-Patent Document 1), only the ion species having a predetermined modified portion can be screened from a sample where a great amount of chemical noises are present, by the precursor ion scan of scanning the mass (m/z) range of the quadrupole mass filter in the pre-stage (Q1) while fixing the mass (m/z) range for the ion detection in the succeeding stage, or neutral loss scan for scanning the mass (m/z) range of the quadrupole mass filter in the pre-stage while fixing the difference of mass between the detection mass (m/z) range in the succeeding stage and the mass (m/z) range in the quadrupole mass filter at the pre-stage. The method is utilized, for example, for confirming the presence of phosphorylated peptide ion species from a specimen where various peptides are mixed.
In order to enhance an extremely low ion utilization efficiency (herein after referred to as Duty Cycle) of the precursor ion scan or neutral loss scan in the prior art, a method of mass selectively ejecting ions from the linear ion trap has been proposed. For instance, U.S. Pat. No. 6,504,148 (Patent Document 4), a method of accumulating ions in a linear ion trap disposed in the pre-stage of a collision chamber, then, introducing only the ions within a specified mass (m/z) range (exactly, at specified mass to charge ratio) from the linear ion trap into the collision reaction chamber to dissociate ions and then detecting the ions by a TOF or quadrupole mass filter thereby improving the Duty Cycle in the neutral loss scan or the precursor scan.
On the other hand, a method of decreasing the space charge of the ion trap is proposed. For example, in the method of the prior art described in US No. 2003/0071206 A1 (Patent Document 5), a quadrupole mass filter is located at the pre-stage of an ion trap and ions other than those required are previously excluded therein. This can introduce only the specified ions as the target for measurement to the ion trap portion, to moderate the space charge of the ion trap.
Further, a method of decreasing the space charge is proposed. For example, in the method of the prior art described in U.S. Pat. No. 5,179,278 (Patent Document 6), a linear ion trap is located to the pre-stage of the 3d quadrupole ion trap and the ions other than those required are excluded in the linear ion trap based on the information such as previously acquired mass spectrum by the application of a supplemental AC voltage. This can introduce only the specified ions as a target for measurement to the 3d quadrupole ion trap portion to moderate the space charge.